Method for increasing the stability of a motor vehicle

ABSTRACT

In a method for increasing the driving stability of a vehicle during braking, compensation steering angles for a regulated and/or controlled steering system are calculated from several input parameters, so that the driving stability of the vehicle is increased by steering interventions. During the steering interventions at least two interference compensation portions for the compensation steering angles are taken into consideration in order to obtain a more comfortable control, from which an interference compensation portion is calculated on the basis of the vehicle course.

BACKGROUND OF THE INVENTION

The invention relates to a method for increasing the driving stabilityof a motor vehicle during braking, in which compensation steering anglesfor a controllable steering system are calculated from several inputparameters, so that the driving stability of the motor vehicle isincreased by steering interventions and an ABS control methodimplementing controlled steering to compensate for a yaw behaviorresulting from different brake effects on the two sides of a vehicle.

In particular, the invention relates to a method for stabilizing a motorvehicle and reducing the stopping distance during braking oninhomogeneous roads with different friction coefficients.

During braking on inhomogeneous roads (i.e. roads with differentfriction coefficients on the left and the right vehicle side)asymmetrical brake forces occur due to the different frictioncoefficients (right-left). These asymmetrical brake forces lead to a yawtorque around the vertical axis of the vehicle which cause the vehicleto carry out a yaw movement towards the road side with the higherfriction coefficient. FIG. 1 represents a vehicle 10 on such aninhomogeneous road.

Vehicles which are not provided with the electronic brake system ABS getinstable in such a driving condition since the cornering force of thetires gets lost when the tires block. The yaw torque resulting from theasymmetrical brake forces causes the vehicle to turn quickly around itsvertical axis towards the side with the high friction coefficient(swerve).

In vehicles provided with the electronic brake system ABS swerving isavoided when braking in such critical situations since the corneringforce of the wheels is maintained by avoiding blocking wheels. However,hereby the yaw torque around the vertical vehicle axis resulting fromthe asymmetrical brake forces is not compensated, but the driver has tocompensate by countersteering. In such critical driving conditions(sudden occurrence of the yaw torque) the ABS control strategy isadapted, as described more in detail in FIGS. 2 a and 2 b, in order notto overstrain the driver. In this case the pressure build-up on thefront axle is controlled during braking in such a way that the pressuredifference on the front axle between the wheel on the high-friction sideand the one on the low-friction side is built up only slowly. This leadsto the fact that the yaw torque around the vertical vehicle axis isbuilt up only slowly so that the driver has enough time forcountersteering (yaw torque limitation on the front axle). At the sametime the rear axle is underbraked in such a way that only the brakepressure of the wheel on the low-friction side is admitted on bothwheels (SelectLow). Thus there is always sufficient cornering potentialon the rear axle and the vehicle can be stabilized easily by thedriver's steering interventions (countersteering). By these two ABSmeasures, yaw torque limitation on the front axle and SelectLow on therear axle the principle pressure developments of which are described inthe FIGS. 2 a and 2 b, very much braking power is given away since thefriction coefficient potential of the high-friction side is not ideallyutilized. This leads to a considerably longer stopping distance which,however, has to be considered as an advantage compared with a vehiclenot provided with ABS which is getting instable.

This delay in building up the yaw torque which leads to a longerstopping distance, can be omitted or reduced when the compensation iscarried out by means of an automatic steering intervention independentof the driver. In this regard DE 40 38 079 A1 describes an at leastpartial compensation of the yaw torque resulting from an ABS control ina μ-split driving condition by that a compensation steering angledepending on the difference of the separately adjusted brake pressuresis set and/or is superimposed on the steering angle defined by thedriver. The autonomous compensation steering angle (automaticcountersteering) improves the maneuverability of the vehicle duringbraking on inhomogeneous roads. For that purpose an active steeringsystem is necessary, i.e. a steering system with which an additionalsteering angle on the wheels can be generated in an active manner andirrespective of the driver's input. This can be achieved, for example,by means of a superimposed steering or a steer-by-wire steering system.

It is the object of the present invention to provide a method and acontrol which improves the maneuverability of the vehicle when brakingon inhomogeneous roads, thus making the vehicle more comfortable.

SUMMARY OF THE INVENTION

According to the present invention, this object is achieved by that incase of braking interventions an interference compensating portion isconsidered for the compensation steering angles which is determined onthe basis of the vehicle course (or the driving condition).

This interference compensating portion is based on the yaw behavior ofthe vehicle and is part of a compensation steering angle demandcomprising at least two interference compensating portions. Here, bymeans of the measuring data which are acquired by sensors and logicallyoperated and analyzed in a model of the driving dynamics control inwhich the data of a motor vehicle may be included, a second interferencecompensating steering angle portion is generated for an active steeringsystem (e.g. a superimposed steering or steer-by-wire steering) bycomparing a nominal yaw signal with an actual yaw signal, the actuatorof the active steering system being adjusted according to a compensationsteering angle demand thus superimposing the steering angle indicated bythe driver. Such active steering systems can be used on the front axleas well as on the rear axle or on all wheels of the vehicle.

The method, in an advantageous manner, includes the determination of afirst interference compensating portion for the compensation steeringangle demand Δδ taking into consideration the brake force differences onthe braked wheels, a second interference compensating portion beingdetermined on the basis of the vehicle course (i.e. driving condition)and the steering angle being modified on the basis of the interferencecompensating portions. In this connection, the first and the secondinterference compensating portions are preferably added up in anadding-up unit and made available to the regulation or control forcorrecting the steering angle input by the driver.

In order to precisely determine the second compensation portion, saidsecond compensation portion should be determined in a device beingprovided with a reference vehicle model circuit in which the inputparameters necessary for determining the vehicle course, i.e. vehiclespeed, steering angle and, if necessary, the friction coefficient, areintroduced which due to the vehicle model in the reference vehicle modelcircuit which simulates the characteristics of the vehicle, determines anominal value for a controlled quantity and in which this nominal valueis compared with a measured value for this controlled quantity in acomparing device, the second compensating portion of the steering angleΔδ_(R) being calculated from the comparative value (controlled quantity)in a driving condition control device. It is an advantage in this casethat the yaw angle speed and/or the lateral acceleration and/or thefloating angle and/or their derivations are determined as a nominalvalue for the controlled quantity.

The determined total compensation steering angle considers the movementof the vehicle in the space (vehicle condition), the compensatingportions being determined from two parameters in such a way that thefirst compensating portion Δδ_(Z) is determined taking intoconsideration an interference yaw torque M_(z) on the basis of differentbrake forces and the second portion Δδ_(R) is determined taking intoconsideration the yaw behavior of the vehicle.

The steering angle correction method is advantageously structured insuch a manner that the first compensating portion is intended to be acontrol portion and the second compensating portion is intended to be acontrol portion.

In this connection the interference yaw torque M_(z) is determined bymeans of a logic operation of the steering lock angle of the steeredwheels, the brake pressures and the rotation behavior of the wheels. Onthe basis of the adjusted brake pressures, the brake forces areadvantageously determined according to the relation{circumflex over (F)}_(x,i)=f{r,B,p_(i),J_(Whl),{dot over (ω)}_(i)}with

-   {circumflex over (F)}_(x,i)=brake force on one wheel i-   r=dynamic wheel radius-   B=brake parameter-   p_(i)=wheel brake pressure-   J_(Whl)=inertial torque of the wheel-   {dot over (ω)}_(i)=rotation acceleration of one wheel i    or    {circumflex over (F)}_(x,i)=f{r,B,p_(i)}.

The interference yaw torque is determined depending on the brake forcesaccording to the relation{circumflex over (M)}_(z)=f{{circumflex over(F)}_(FL),s_(FL),{circumflex over (F)}_(FR),s_(FR),l_(F),{circumflexover (F)}_(RL),s_(RL),{circumflex over (F)}_(RR), s_(RR),δ}with

-   {circumflex over (F)}_(FL)=brake force at the front on the left-   s_(FL)=half the tread of the left front wheel-   {circumflex over (F)}_(FR)=brake force at the front on the right-   s_(FR)=half the tread of the right front wheel-   l_(F)=distance of the front axle from the center of gravity-   {circumflex over (F)}_(RL)=brake force at the rear on the left-   s_(RL)=half the tread of the left rear wheel-   {circumflex over (F)}_(RR)=brake force at the rear on the right-   s_(RR)=half the tread of the right rear wheel-   δ=steering lock angle of the steered wheels

In order to improve the dynamics of the steering angle correction methodthe compensation gain K_(FFW) and K_(FB) of the single fed backcontrolled quantities should be adjusted depending on the drivingbehavior of the vehicle and the environmental conditions.

The controlling portion Δδ_(z) of the steering angle demand Δδ isdetermined according to the relation Δδ_(z)=K_(FFW)(Δ{overscore(p)},v)*M_(z) on the basis of the determined acting interference yawtorque. In this case the average friction coefficient potential of thehigh-friction coefficient side and the low-friction coefficient sidecorresponds to the average brake pressure on the front axle if bothfront wheels are controlled by the ABS system thus fully exploiting thefriction coefficient available in the single case. Here the compensationgain K_(FFW)(Δ{overscore (p)},v) taking into consideration the availableaverage friction coefficient potential and the vehicle speed, determinedby means of the rotation behavior of the wheels in the form of a vehiclereference speed is adapted by way of the average brake pressure of thefront axle.

In another advantageous embodiment the second compensating portionΔδ_(R) of the steering angle demand Δδ is determined by a P portionΔδ_(R,P) based on the yaw rate deviation Δ{dot over (ψ)} and a D portionbased on the yaw acceleration deviation Δ{umlaut over (ψ)}. Here the Pportion is determined on the basis of the relationΔδ_(P,R)=K_(FB,P)(v)*Δ{dot over (ψ)}. The gain factor K_(FB,P)(v) forthe adaptation of the controlled quantity yaw rate deviation Δ{dot over(ψ)} depends on the vehicle speed which is determined by the rotationbehavior of the wheels in the form of a vehicle reference speed.

The D portion is determined in an advantageous manner according to therelation δ_(R,D)=K_(FB,D)(v)*Δ{umlaut over (ψ)}, where the gain factorfor the feedback of the controlled quantity yaw acceleration deviationΔ{umlaut over (ψ)} depends on the vehicle speed.

The method for increasing the driving stability of a motor vehicleincludes at least one ABS control function in order to be able todevelop an ABS control method in which a driving condition caused bybraking operations with different brake pressures or brake forces on thesingle wheels and defined by the determined brake force difference, insuch a favorable way that the instabilities caused by the drivingcondition can at least in part be compensated by an intervention in anopen-loop or closed-loop controlled steering system. In this case it isan advantage that the ABS control function is a part of an ESP control.

The correction of the steering angle is admitted if a driving conditionwith different friction coefficients on each side (μ-Split) has beenrecognized. The recognition of a driving condition or a course where thedeviation between the vehicle movement and the driver's input is causedby different brake pressures or forces is determined and steeringinterventions are admitted if at least the following conditions aresatisfied:

-   -   If it is recognized that the vehicle is driving straight ahead:    -   a) stop light switch signal present and    -   b) stop light switch sensor in working order and    -   c) brake actuation by the driver has been recognized by means of        TMC pressure and    -   d) driving forward has been recognized and    -   e) one of the following conditions is also satisfied        -   e1) if one front wheel is controlled by the ABS system for a            certain period of time and the other front wheel is not            controlled by the ABS system or        -   e2) if both front wheels are in the first ABS cycle and the            pressure difference on the front axle exceeds a limit value            or        -   e3) if both front wheels are controlled by the ABS system            for a certain period of time and at least one front wheel            shows a certain minimum ABS blocking pressure and one            blocking pressure exceeds the blocking pressure of the other            wheel by more than a certain limit value.    -   If it is recognized that the vehicle is cornering:    -   a) Stop switch signal present and    -   b) stop switch sensor in working order and    -   c) brake actuation by the driver has been recognized by means of        TMC pressure and    -   d) driving straight ahead has been recognized and    -   e) one of the following conditions is also satisfied        -   e1) the curve outer front wheel is controlled by the ABS            system before the curve inner front wheel or if for a            certain period of time        -   e2) both front wheels are being controlled by the ABS system            and at least one front wheel shows a certain minimum ABS            blocking pressure and the blocking pressure of the curve            inner wheel exceeds the blocking pressure of the curve outer            wheel by more than a certain limit value.

For deactivating the steering angle correction method, at least one ofthe following requirements must be satisfied so that the active steeringinterventions are finished:

a) no front wheel is being controlled by the ABS system or

b) there is no stop switch signal or

c) the stop switch sensor is defective or

d) the brake actuation by the driver is not recognized (no TMC pressurepresent).

In μ-Split driving conditions the ABS brake pressure control canpreferably be modified by means of the ABS control method. According tothe present invention an ABS brake pressure control with single wheelcontrol is to be provided at least on one vehicle axle in which thedeviation between the vehicle movement and the driver's control inputoccurring with the ABS control due to the different friction coefficienton the two vehicle sides is compensated at least in part by that acompensation steering angle is determined and is superimposed on thevehicle steering angle. The ABS brake pressure control is characterizedby the following steps:

admission of high pressure build-up gradients on the wheel with a highfriction coefficient.

admission of pressure differences on the rear axle according to therelation Δp_(HA)=f({dot over (ψ)},δ_(Whl),v,a_(y)).

In order to maintain the ABS control function in each and everysituation thus maintaining the maneuverability of the vehicle duringbraking with high brake pressure, it is intended that the conventionalABS control strategy is used in case of a breakdown of the controlled orregulated steering system.

One device includes a driving dynamics controller with at least one ABSfunction, preferably an ESP and ABS function, which is connected with anopen-loop and/or a closed-loop control for correcting the steering, thedevice being built in such a way that it includes a first determinationunit for determining the steering angle defined by the driver

a second determination unit for determining an interference compensationsteering angle on the basis of brake forces and/or brake pressure or aninterference yaw torque,

a third device for determining an interference compensation steeringangle on the basis of the yaw behavior of the vehicle and

a logic unit for linking the first and the second interferencecompensation steering angle in order to obtain a compensation steeringangle demand.

The following advantages result from the methods and devices:

-   -   The driver is relieved by automatic countersteering of the        control system so that he, in the ideal case, does not have to        correct anything.    -   The substantial advantage of the division into interference        compensation and superimposed control of the driving condition        is that by means of the interference parameter overlay it is        possible to immediately react to the interference and not only        when the vehicle tends to become instable. The superimposed        control of the driving condition improves the total behavior of        the vehicle, and interferences which cannot be compensated by        the simple control (interference compensation) are hereby        eliminated by control.    -   By recognizing the situation far more quickly than the driver        and by countersteering far more quickly, the electronic brake        system ABS can much better exploit the friction coefficient        potential on the single wheels. For this reason the ABS        strategies on inhomogeneous roads are adapted in such a way that        on the front axle a much quicker pressure build-up on the wheel        of the side with the high friction coefficient is admitted, and        on the rear axle a pressure difference depending on the steering        lock angle, the driving speed and the driving condition        parameters (e.g. yaw rate or lateral acceleration) is admitted        (softened SelectLow). By better exploiting the friction        coefficient potentials, particularly on the side with a high        friction coefficient, the stop distances are much shorter.    -   By using the control system in combination with an active        steering system and by means of the adapted ABS control        strategies, the conflict between the countersteering expenditure        and the brake distance prolongation occurring during brake        operations on inhomogeneous friction coefficients can be solved        to a high degree. For the driver, there are significant        advantages in terms of safety (shorter stop distance and vehicle        stability) as well as in terms of comfort (considerably less        expenditure for the driver to countersteer).

One embodiment of the present invention is described more in detail inthe following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures,

FIG. 1 shows a schematic representation of the asymmetric brake forcesof a vehicle and the resulting interference yaw torque of a μ-Splitroad,

FIG. 2 a shows the pressure development on the front axle in case ofactive yaw torque limitation according to the state of the art,

FIG. 2 b shows the pressure development on the rear axle with activeSelectLow according to the state of the art,

FIG. 3 shows a block diagram representing the control system withinterference parameter overlay and superimposed control of the drivingcondition,

FIG. 4 shows a block diagram representing the interference parameteroverlay with an estimation of the interference yaw torque,

FIG. 5 shows a block diagram representing the superimposed control ofthe driving condition,

FIG. 6 shows a block diagram representing the determination of thepressure difference on the rear axle on the basis of the drivingdynamics condition of the vehicle,

FIG. 7 a shows the pressure development on the front axle with adaptedyaw torque limitation according to the present invention,

FIG. 7 b shows the pressure development on the rear axle due to amodification of the SelectLow according to the present invention,

FIG. 8 shows a representation of the vehicle geometry, and

FIG. 9 shows a representation of the ABS control cycle.

DETAILED DESCRIPTION OF THE DRAWINGS

The steering lock angle necessary for the automatic countersteering isdetermined by a calculating unit 30 (FIG. 3) which composes the steeringlock angle on the basis of two portions (interference parameter overlayand superimposed driving control).

The first portion results from the interference parameter overlay orinterference parameter compensation of the interference yaw torque{circumflex over (M)}_(z) caused by the asymmetric brake forces duringbraking. This interference yaw torque is first estimated in adetermination unit 40, schematically represented in FIG. 4, based on thebrake pressure information of the single wheels, according to theequations 1 and 2. The input parameters introduced into thedetermination unit are the wheel brake pressures p_(i), the wheelrotation speed ω_(i), and the wheel locking angle feedback δ_(WHL). Anelectronic brake system is necessary for determining the wheel brakepressures, which either estimates or observes the brake pressures on thesingle wheels on the basis of the model and measures the brake pressuresof the single wheels by means of pressure sensors, or a brake-by-wiresystem (EHB/EMB) which bases on these parameters. The determination ofthe interference yaw torque according to equation 2 bases on brakeforces {circumflex over (F)}_(x,i) on the wheels. The brake forcescan—as indicated in equation 1—be calculated essentially on the basis ofthe brake pressure information. Alternatively, systems can be used whichdirectly measure the brake forces (e.g. side panel torsion sensor, hubsand similar). The steering lock angle δ_(Z) which depends on the drivingparameters (e.g. vehicle speed, brake pressure difference between highand low friction coefficient, average brake pressure level etc.) and isnecessary for compensating the interference yaw torque (FIG. 4) iscalculated in an adaptive manner from the estimated interference yawtorque. With regard to the lateral dynamics, the interference parameteroverlay functions as a mere control. This results in that theinterference yaw torque is not compensated ideally in all cases since itis superimposed by other interferences and inaccuracies which are notcaptured. Inaccuracies may occur, for example, due to changes of thebrake disk friction coefficient.

Thus the interference parameter overlay—as represented in FIG. 3—issuperimposed by a driving controller 50. This driving controller whichis represented in FIG. 5 and will be described more in detail later on,defines an additional steering lock angle δ_(R) on the basis of thedriving parameters, such as yaw rate and optionally in addition also thelateral acceleration or the floating angle of the vehicle. Device 50,i.e. the controller, works in an adaptive manner, i.e. the control gainof the single fed back driving conditions is adapted e.g. on the basisof the vehicle speed v.

These two steering angle actuating demands (resulting from theinterference parameter overlay and the superimposed driving control) arepreferably summed up in a adding unit 31 and adjusted by the activesteering system in the form of a steering lock angle δ_(WHL). Thedetermination of the steering lock angle δ_(WHL) necessary for thestabilization and the adjustment of the steering lock angle occur muchquicker than an average driver can recognize the situation and react bycountersteering. This quick reaction of the control system and theactive steering system allows the electronic brake system ABS to beadapted in such a way that the friction coefficient potential on thesingle wheels (in particular on the high friction coefficient side) canbe exploited much better).

To this end the control strategies of the ABS system on inhomogeneousfriction coefficients are modified:

The yaw torque limitation on the front axle is considerably reduced sothat a big pressure difference quickly builds up between the wheel onthe high friction coefficient side and the one on the side with a lowfriction coefficient (high pressure build-up gradient on the wheel witha high friction coefficient). Nearly contemporarily to the build-up ofthe pressure difference, a yaw torque around the vertical vehicle axisis generated. Due to the estimated interference yaw torque resultingfrom the brake pressure information according to the equations 1 and 2or by means of a system measuring directly the wheel forces, thecontroller immediately countersteers, even before the driver canrecognize the situation on the basis of the yaw behavior of the vehicle.A second measure to obtain a better brake performance is to modify alsoSelectLow in such a way that a pressure difference is admitted also onthe rear axle. However, this pressure difference is not always admitted,but depends on the steering angle, which is restricted by the vehiclespeed and the driving parameters (equation 3, FIG. 6). If the steeringlock angle points toward the side with the low friction coefficient andif the vehicle turns towards the side with the low friction coefficient,a pressure difference is admitted on the rear axle. This leads to ahigher brake force on the side with the high friction coefficient, theinterference yaw torque increases and at the same time the lateral forcepotential on this wheel is reduced. Due to the bigger interference yawtorque, the rotation to the side with the low friction coefficient stopsand the vehicle begins to turn towards the side with the high frictioncoefficient. By turning towards the side with the high frictioncoefficient the admitted pressure difference on the rear axle and thusthe brake force on the side with the high friction coefficient arereduced at the same time, which again leads to more side force potentialon the rear wheel on the side with the high friction coefficient. Bythis and by the superimposed steering interventions of the controlsystem interacting with the active steering system, the vehicle isstabilized. Nevertheless the driver can steer towards the side with thehigh or the low friction coefficient, according to his steering input.The pressure difference on the rear axle admitted by the drivingdynamics weakening of the SelectLow is limited to a maximum pressuredifference so that the rear wheel on the side with the high frictioncoefficient does not lose too much side force potential. In case of highspeed or increasing lateral acceleration this maximum admitted pressuredifference on the rear axle can be reduced to zero (corresponds toSelectLow).

These modifications in the ABS control strategy (high pressure build-upgradient for the yaw torque limitation on the front axle as well assoftened SelectLow on the rear axle according to the driving condition)lead to an essentially better utilization of the available frictioncoefficient potential. Hereby the brake distance can be reducedsignificantly.

When the active steering system fails, the conventional ABS controlstrategy (yaw torque limitation and SelectLow) is utilized.

The steering angle correction system works as follows:

The method of correcting the compensating steering angle is activated onthe basis of a recognized μ-split situation. According to anadvantageous embodiment, the recognition of a μ-split driving conditionis based on the following sensor signals:

-   -   stop light switch signal (SLS)    -   pressure sensor signal of the tandem main cylinder (TMC)    -   pressure sensor signals of the wheel brake circuit    -   wheel rotation sensors    -   yaw rate sensor(s)    -   lateral acceleration sensor(s)    -   internal ESP condition (ESP signals regarding ESP interventions)

The distinction between driving straight ahead and cornering (right orleft-hand curve) is made by means of the yaw rate and the lateralacceleration. Depending on whether the vehicle is driving straight aheador is cornering, the following signals must be present in order toactivate the correction of the compensating steering angle:

The μ-split driving condition is recognized as follows when drivingstraight ahead:

-   -   stop light switch signal (SLS) is present, stop light switch        sensor is in working order, braking actuation by the driver is        recognized by means of the TMC pressure, driving forward is        recognized and at least one front wheel is controlled by the ABS        system, or    -   if after exceeding one first time-dependent limit value one        front wheel is controlled by the ABS system and the other front        wheel is not controlled by the ABS system    -   or if both front wheels are in the first ABS cycle and the        pressure difference on the front axle exceeds a first        pressure-dependent limit value, or    -   if after exceeding a second time-dependent limit value both        front wheels are controlled by the ABS system and at least one        front wheel shows an ABS blocking pressure which exceeding a        second pressure-dependent limit value and the ABS blocking        pressure on a front wheel corresponds to at least x times the        blocking pressure of the other front wheel.

A μ-split driving condition which has been recognized when drivingstraight ahead is reset as follows:

The ABS system does not control any front wheel or there is no SLS orthe SLS sensor is defective or the brake actuation by the driver is notrecognized or

there is an SLS and the SLSS sensor is in working order and the brakeactuation by the driver is recognized and after exceeding atime-dependent limit value the ABS blocking pressure on both frontwheels is smaller than a pressure-dependent limit value or the ABSblocking pressure on one front wheel does no longer correspond to atleast x times the blocking pressure of the other front wheel.

During cornering the μ-split driving condition is recognized as follows:

-   -   the stop light switch signal (SLS) is present, stop light switch        sensor is in working order, brake actuation by the driver is        recognized by means of the TMC pressure, driving forward is        recognized and at least one front wheel is controlled by the ABS        system and    -   the curve outer front wheel is controlled by the ABS system        before the curve inner front wheel, or    -   if both front wheels are controlled by the ABS system for more        than a preset period of time and at least one front wheel shows        an ABS blocking pressure exceeding a limit value and the ABS        blocking pressure on a curve inner front wheel corresponds to at        least x times the blocking pressure of the curve outer front        wheel.

A μ-split driving condition which has already been recognized duringcornering is reset as follows:

-   -   The ABS system does not control any front wheel or there is no        SLS or the SLS sensor is defective or the brake actuation by the        driver is not recognized, or    -   there is an SLS and the SLS sensor is in working order and the        brake actuation by the driver is recognized and the ABS blocking        pressure on both front wheels is smaller than a limit brake        value for more than the preset period of time or the ABS        blocking pressure on the curve inner front wheel does no longer        correspond to at least x times the blocking pressure of the        curve outer front wheel.        Compensating Steering Requirement

In order to activate the compensating steering demand, the μ-splitdriving condition must have been recognized and the compensatingsteering demand must have been activated as described above. Thesteering angle demand Δδ is based on two portions: the first portionΔδ_(Z) is defined by means of the interference parameter compensation(control portion) compensating the acting interference yaw torque. Thiscontrol portion is superimposed by a control portion Δδ_(R) based on theyaw behavior of the vehicle. The two portions described in the following(control and control portion) are summed up resulting in the totalsteering demand Δδ toΔδ=Δδ_(Z)+Δδ_(R).

The steering demand is based on the following sensor signals:

-   -   pressure sensor signals in each wheel brake circuit    -   yaw rate signals    -   nominal steering angle signals “driver's steering angle demand”    -   total steering angle signals on the wheel    -   wheel rotation speed sensor signals    -   lateral acceleration signals    -   SLS signals    -   pressure sensor signals of the TMC    -   ESP condition (ESP interventions)    -   ESP condition (reset of the single-track model)        Open-Loop Control Portion (Interference Parameter Compensation)

The control portion of the steering demand corresponds to aninterference parameter compensation. In this case the interference yawtorque M_(z) acting as interference parameter and resulting from theasymmetrical brake forces, is compensated to a high degree by directfeedback from the compensation gain K_(FFW)({overscore (p)}_(FA),v). Theestimated interference yaw torque is the direct input parameter for theadditional steering angle demand Δδ_(Z) of the control portion (FFW=FeedForward Control). The following relation appliesΔδ_(Z) =K _(FFW)({overscore (p)} _(FA) ,v)·M _(z)·

The interference yaw torque is estimated by means of the cinematic rigidbody relations on the basis of the brake forces of the single wheels andthe steering angle lock of the front wheels. The static brake forces ofthe single wheels are defined on the basis of the ABS blocking pressuresof the single wheels and the dimensions of the wheel brake.Additionally, the wheel accelerations must be considered in order tocalculate the dynamic brake forces. The definition of the ABS blockingpressures is described later on.

The compensation gain factor K_(FFW)({overscore (p)}_(FA),v) is adaptedby way of the average brake pressure on the front axle. If both frontwheels are controlled by the ABS system, the average brake force on thefront axle corresponds to the total (average of left and right vehicleside) available friction coefficient potential. This frictioncoefficient potential again influences the compensating steering anglewhich can be set with the active steering.

In case of small driving speeds (between 10 and 2 km/h), the additionalsteering angle demand Δδ_(Z) is faded off in a linear manner up toΔδ_(Z)=0.

Summary Control Portion:

The steering portion based on the interference parameter compensationdepends basically on the steering angle lock of the front wheels and theABS blocking pressures which are based—as is described lateron—essentially on the pressure sensor signals and the ABS phaseinformation (defined from the wheel rotation speed sensor signals).

Closed-Loop Control Portion

The control portion Δδ_(R) of the steering demand based on the yawbehavior of the vehicle, consists of a P portion Δδ_(R,P) (controlledquantity yaw rate deviation) and a D portion Δδ_(R,D) (controlledquantity yaw acceleration deviation). The P and D portions described inthe following are added resulting in the total control portion Δδ_(R) asfollows:Δδ_(R)=Δδ_(R,P)+Δδ_(R,D).P Portion (Yaw Rate Deviation)

The controlled quantity for the P portion corresponds to the yaw ratedeviation Δ{dot over (ψ)}. For the steering demand portion resultingfrom the P portion, the following control law appliesΔδ_(R,P) =K _(FB,P)(v)·Δ{dot over (ψ)}.

The yaw rate deviation Δ{dot over (ψ)} is defined as difference betweenthe measured yaw rate of the vehicle {dot over (ψ)}_(ist) and thereference yaw rate of the vehicle {dot over (ψ)}_(ref) (single-trackmodel) defined on the basis of the driver's direction input (driver'ssteering angle including variable steering ratio) thus resulting inΔ{dot over (ψ)}={dot over (ψ)}_(ist)−{dot over (ψ)}_(ref).

The actual yaw rate of the vehicle {dot over (ψ)}_(ist) is measureddirectly with a yaw rate sensor. The yaw rate sensor is combined with alateral acceleration sensor in a sensor cluster in which the yaw rate aswell as the lateral acceleration with redundant sensor elements aremeasured.

The reference yaw rate of the vehicle {dot over (ψ)}_(ref) is defined bymeans of a single-track model of the vehicle. The most important inputparameters for the one-track model are the manual driver input (driver'ssteering angle including variable steering ratio portions) and thevehicle speed. The actual friction coefficient of the road surface isdefined by means of the measured lateral acceleration and the resultingfriction coefficient potential is considered in the one-track model whencalculating the reference yaw rate.

The gain factor K_(FB,P)(v) for the controller feedback of the yaw ratedeviation Δ{dot over (ψ)} is adapted by way of the actual vehicle speedv. Since the vehicle speed influences the driving behavior of thevehicle in a significant manner, this is considered in the controllergain and thus also in the circuit closed by way of the controller of thevehicle.

D Portion (Yaw Acceleration Deviation)

The controlled quantity for the D portion corresponds to the yawacceleration deviation Δ{umlaut over (ψ)}. For the steering demandportion resulting from the D portion, the following appliesΔδ_(R,D) =K _(FB,D)(v)·Δ{umlaut over (+104)}.

The yaw acceleration deviation Δ{umlaut over (ψ)} is determined bydifferentiating the yaw rate deviation Δ{umlaut over (ψ)}.${\Delta\overset{¨}{\psi}} = {{\frac{\mathbb{d}}{\mathbb{d}t}\Delta\overset{.}{\psi}} = {\frac{\mathbb{d}}{\mathbb{d}t}{( {{\overset{.}{\psi}}_{ist} - {\overset{.}{\psi}}_{ref}} ).}}}$

The yaw acceleration deviation is thus based on the same signal sourcesas the yaw rate deviation: measured yaw rate of the vehicle {dot over(ψ)}_(ist) and reference yaw rate of the vehicle {dot over (ψ)}_(ref)which depends immediately from the driver's direction input (driver'ssteering angle including variable steering ratio portions) and thevehicle speed. (Consideration of the actual friction value of the roadby means of the measured lateral acceleration).

The gain factor K_(FB,D)(v) for the controller feedback of the yawacceleration deviation Δ{umlaut over (ψ)} is adapted by way of theactual vehicle speed. Since the vehicle speed influences the drivingbehavior of the vehicle in a significant manner, this is considered inthe controller gain and thus also in the control circuit of the vehicleclosed by the controller.

Summary Control Portion

The control portion Δδ_(R) is based essentially on the signal of the yawrate sensor {dot over (ψ)}, the driver's steering angle demand δ_(DRV)including variable steering ratio and the vehicle speed v which is basedon the signals of the wheel rotation speed sensors.

Calculation of the ABS Blocking Pressure

The brake pressure on the wheel is defined as ABS blocking pressurewhich causes the wheel tending to block. If the friction coefficientduring an ABS braking operation is nearly homogenous, the brake pressureon the wheel oscillates around the ABS blocking pressure. The ABSblocking pressure is determined individually for each wheel in thefollowing manner:

If the wheel is not in the first ABS control cycle and the ABS systemdetermines that the wheel is instable thus tending to block (ABS phase2) and if the wheel in the preceding control loop was not yet in phase 2or phase 4, then at least 85%, preferably 95%, of the actual wheelpressure are frozen as ABS blocking pressure of the wheel. If the wheelis neither controlled by the ABS system nor in the first ABS controlcycle, the wheel pressure is used instead of the ABS blocking pressure.If the wheel is controlled by the ABS system, but is not in phase 2, themaximum of the last ABS blocking pressure and 95% of the wheel pressureis used since in pressure build-up phases the wheel pressure may exceedthe last ABS blocking pressure. If a wheel is instable for more than aperiod of time between 90 and 110 ms (phase 2) the ABS blocking pressureis no longer used, but the wheel pressure, since the wheel pressure hasdeviated too much from the ABS blocking pressure due to the continualpressure reduction.

If the wheel pressure amounts to less than 50% of the last ABS blockingpressure or if the brake slip of the wheel corresponds to more than 50%,the wheel pressure is taken again (recognition of a friction coefficienttransition from high friction coefficients to low frictioncoefficients).

If the ESP system intervenes on a wheel, the ABS blocking pressure isnot adapted, but maintained constant.

If the driver does not brake anymore, the ABS blocking pressures arereset to zero.

Summary:

The determination of the blocking pressure is based essentially on thepressure sensor signals and the necessary ABS phase information is basedessentially on the wheel rotation speed sensors.

ABS Phase Information and ABS Control Cycle: ABS phase Wheel conditionABS action Phase 0 no ABS control unpulsed pressure build-up Phase 1 noABS control, pulsed pressure build-up from 0 insignificant wheeldynamics Phase 2 instable wheel, high pressure reduction amount of slipat the wheel Phase 4 instable wheel, wheel maintain pressure, leaves theslip range pulsed pressure build-up Phase 3 stable wheel, low slip onpulsed pressure build-up the wheel Phase 1 wheel shows insignificantmaintain pressure from 3 dynamics Phase 5 wheel is spinning unpulsedpressure build-up from 0 Phase 5 wheel is spinning unpulsed pressurebuild-up from 3Equations:

1. Estimation of the brake forces on the basis of the brake pressures:

-   -   Equation showing the factors influencing a wheel neglecting        driving torque and assuming that the wheel contact force applies        in the wheel contact point        J _(Whl){dot over (ω)}_(i) =M _(br,i) +F _(x,i) r _(Whl).    -   This results together with a brake torque of M_(br,i)=B*p_(i)        for the estimation of the circumferential force {circumflex over        (F)}_(x,i) from brake pressure and wheel acceleration in        ${\hat{F}}_{x,i} = {{\frac{1}{r}B^{*}p_{i}} + {\frac{1}{r}J_{Whl}{{\overset{.}{\omega}}_{i}.}}}$    -   In case of lower accuracy requirements the dynamic portion        $\frac{1}{r}J_{Whl}{\overset{.}{\omega}}_{i}$    -   may be neglected. Stationarily the brake force results in        ${\hat{F}}_{x,i} = {\frac{1}{r}B^{*}p_{i}}$

2. Estimation of the interference yaw torque from the brake forces

-   -   For vehicles with front wheel steering with the steering angle        lock δ and the vehicle geometry according to FIG. 8 the        interference yaw torque results in        {circumflex over (M)} _(z)=cos(δ)└{circumflex over (F)} _(FL) s        _(FL) −{circumflex over (F)} _(FR) s _(FR)┘−sin(δ)└{circumflex        over (F)} _(FL) l _(F) +{circumflex over (F)} _(FR) l _(F)        ┘+{circumflex over (F)} _(RL) s _(RL) −{circumflex over (F)}        _(RR) s _(RR).

3. SelectLow:

-   -   On the rear axle a pressure difference is admitted which depends        on the driving dynamics condition. For the admitted pressure        difference on the rear axle applies        Δp _(HA) =f({dot over (ψ)},δ_(Whl) ,v,a _(y)).

1-23. (canceled)
 24. A method for increasing the driving stability of amotor vehicle during braking, in which compensation steering angles fora controllable steering system are calculated from several inputparameters, so that the driving stability of the motor vehicle isincreased by steering interventions, wherein in the steeringinterventions an interference compensation portion is considered in thecalculation of the compensation steering angles on the basis of thedriving condition of the vehicle.
 25. The method according to claim 24,comprising the steps of determining a first interference compensationportion of the compensating steering angle demand Δδ taking intoconsideration brake force differences on braked wheels of the vehicle,and determining a second interference compensation portion from thedriving condition of the motor vehicle and modification of the steeringangle depending on interference compensation portions.
 26. The methodaccording to claim 25, wherein the second compensation portion iscalculated in a device including a reference vehicle model circuit inwhich the input parameters necessary for defining the course, such asvehicle speed, steering angle, and—if necessary—friction coefficient,are introduced, which due to a vehicle model being built in thereference vehicle model circuit and simulating the characteristics of avehicle defines a nominal value for a controlled quantity and in whichthis nominal value is compared with a measured value for this controlledquantity in a comparative unit, calculating the second compensationportion of the steering angle Δδ_(R) on the basis of the comparativevalue representing the controlled quantity.
 27. The method according toclaim 26, wherein one element out of the group consisting of yaw anglespeed, lateral acceleration, and floating angle, is defined as nominalvalue for the controlled quantity.
 28. The method according to claim 25,wherein the first compensation portion Δδ_(Z) is determined taking intoconsideration an interference yaw torque M_(z) calculated from differentbrake forces and the second portion Δδ_(R) is determined taking intoconsideration vehicle yaw behavior.
 29. The method according to claim25, wherein the first compensation portion is an open-loop controlportion and the second compensation portion is a closed-loop controlportion.
 30. The method according to claim 24, wherein the interferenceyaw torque M_(z) is calculated from a logical operation of a steeringlock angle of steered wheels, of brake forces and of rotational behaviorof the wheels.
 31. The method according to claim 30, wherein the brakeforces are calculated from brake pressures on the basis of the relation{circumflex over (F)}_(x,i)=f{r,B,p_(i),J_(Whl),{dot over (ω)}_(i)} with{circumflex over (F)}_(x,i)=Brake force on one wheel i r=dynamic wheelradius B=Brake parameter p_(i)=Wheel brake pressure J_(Whl)=Moment ofinertia of the wheel {dot over (ω)}_(i)=Rotation acceleration of a wheeli or{circumflex over (F)}_(x,i)=f{r,B,p_(i)}.
 32. The method according toclaim 31, wherein the interference yaw torque is calculated on the basisof the relation{circumflex over (M)}_(z)=f{{circumflex over(F)}_(FL),s_(FL),{circumflex over (F)}_(FR),s_(FR),l_(F),{circumflexover (F)}_(RL),s_(RL),{circumflex over (F)}_(RR),s_(RR),δ} with{circumflex over (F)}_(FL)=Brake force on the front at the lefts_(FL)=half the tread of the left front wheel {circumflex over(F)}_(FR)=Brake force on the front at the right s_(FR)=half the tread ofthe right front wheel l_(F)=Distance between front axle and center ofgravity {circumflex over (F)}_(RL)=Brake force on the rear at the lefts_(RL)=half the tread of the left rear wheel {circumflex over(F)}_(RR)=Brake force on the rear at the right s_(RR)=half the tread ofthe right rear wheel δ=Wheel steering lock of the steered wheels
 33. Themethod according to claim 24, wherein compensation gains K_(FFW) ofsingle feed-back controlled quantities are adapted depending on thedriving behavior of the vehicle and possibly the environmentalconditions.
 34. The method according to claim 25, wherein the secondcompensation portion Δδ_(R) of the steering angle compensation Δδ iscalculated from a P portion Δδ_(R,P) based on a vehicle yaw ratedeviation Δ{dot over (ψ)} and a D portion Δδ_(R,D) based on a yawacceleration deviation Δ{umlaut over (ψ)}.
 35. The method according toclaim 34, wherein the P portion is calculated according to the relationΔδ_(R,P) =K _(FB,P)(v)*Δ{dot over (ψ)}.
 36. The method according toclaim 35, wherein the gain factor K_(FB,P)(v) for the feedback of thecontrolled quantity of the yaw rate deviation Δ{dot over (ψ)} depends onthe vehicle speed.
 37. The method according to claim 34, wherein the Dportion is calculated according to the relationδ_(R,D) =K _(FB,D)(v)*Δ{umlaut over (ψ)}.
 38. The method according toclaim 34, wherein the gain factor K_(FB,D)(v) for the feedback of thecontrolled quantity yaw acceleration deviation Δ{umlaut over (ψ)}depends on the vehicle speed v.
 39. An ABS control method in which a yawbehavior resulting from brake actuations with different brake pressuresor forces on the single wheels, which is defined from the determinedbrake force difference, is at least in part compensated by theintervention of a controlled steering system, wherein in the steeringinterventions an interference compensation portion is considered in thecalculation of the compensation steering angles on the basis of thedriving condition of the vehicle.
 40. The ABS control method accordingto claim 39, wherein a driving condition is determined by means of a yawbehavior resulting from different braking pressures or forces andadmitting steering interventions, if at least the following conditionsare satisfied with recognized straight-forward driving or cornering: a)stop light switch signal present and b) stop light switch sensor inworking order and c) brake actuation by the driver recognized by meansof the TMC pressure and d) driving straight forward is recognized. 41.The ABS control method according to claim 39, wherein, after it isrecognized that the vehicle is driving approximately straight forward,compensation steering angles are calculated if at least one of thefollowing conditions is satisfied: e1) if one front wheel is controlledby the ABS system for a certain period of time and the other front wheelis not controlled by the ABS system or e2) if both front wheels are in afirst ABS cycle and the pressure difference on the front axle exceeds alimit value or e3) if both front wheels for a certain period of time arecontrolled by the ABS system and at least one front wheel shows acertain minimum ABS blocking pressure and one blocking pressure exceedsthe blocking pressure of the other wheel by more than a limit value. 42.The ABS control method according to claim 39, wherein, after it isrecognized that the vehicle is cornering, compensation steering anglesare calculated if at least one of the following conditions is satisfied:e1) the curve outer front wheel is controlled by the ABS system beforethe curve inner wheel or e2) if for a certain period of time both frontwheels are controlled by the ABS system and at least one front wheelshows a certain minimum ABS blocking pressure and the blocking pressureof the curve inner wheel exceeds the blocking pressure of the curveouter wheel by more than a certain limit value.
 43. The ABS controlmethod according to claim 39, wherein at least one of the followingrequirements must be satisfied in order to terminate the steeringinterventions: a) no front wheel is controlled by the ABS system, b)there are no stop light switch signals, c) the stop light switch sensoris defective, or d) the brake actuation by the driver is not.
 44. An ABSbrake pressure control method for a two-axle, four-wheel vehicle withsingle-wheel control on at least one vehicle axle in which due to adifferent friction coefficient on the two vehicle sides a yaw behavioroccurring during ABS control is compensated at least in part bycalculating a compensation steering angle for steering control andsuperimposing it on the vehicle steering angle, comprising the steps ofadmitting high pressure build-up gradients on a wheel with a highfriction coefficient admitting pressure differences on the rear axleaccording to the relationΔp _(HA) =f({dot over (ψ)},δ_(Whl) ,v,a _(y)).
 45. The ABS brakepressure control according to claim 44, wherein a traditional ABScontrol strategy is used if the steering control fails.
 46. A drivingdynamic controller with at least an ESP and an ABS function beingconnected to a controller and/or a control for correcting the steering,comprising a first determination unit for determining a steering anglerequired by the driver a second determination unit for determining aninterference compensation steering angle on the basis of brake forces athird device for determining an interference compensation steering angleon the basis of the vehicle yaw behavior and a logic unit for linkingthe first and the second interference compensation steering angle with acompensation steering angle demand.